RADIO RESOURCE MANAGEMENT IN WiMAX

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MEE09:16

RADIO RESOURCE

MANAGEMENT IN WiMAX

Noman Shabbir Hasnain Kashif

This thesis is presented as part of Degree of Master of Science in Electrical Engineering

Blekinge Institute of Technology March 2009

Blekinge Institute of Technology School of Engineering

Department of Signal Processing

Supervisor: Prof. Dr.-Ing. Hans-Jürgen Zepernick Examiner: Prof. Dr.-Ing. Hans-Jürgen Zepernick

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ABSTRACT

Today, different types of cellular networks are actively working on the radio links. For instance, the Global System for Mobile Communication (GSM) is being used in nearly two hundred countries and currently it has around two and half billion users all over the world. Universal Mobile Telecommunication System (UMTS) is currently deployed in many countries and it is providing increased data rates, coverage and mobility as compared to GSM. Wireless Local Area Networks (WLAN) are very famous when we have a small area and none real time services. Worldwide Interoperability for Microwave Access (WiMAX) is a new technology and it is in deployment phase. In all these cellular technologies, we have very limited recourses and we have to make best use of them by proper management. Radio Resource Management (RRM) is a control mechanism for the overall system which is being used to manage radio resources in the air interface inside a cellular network. The main objective is to utilize the available spectral resources as efficiently as possible. Our aim is to use them in the best possible way to maximize the performance and spectral efficiency in such a way that we have maximum number of users in our network and Quality of Service (QoS) is up to the mark. In a cellular communication system, a service area or a geographical region is divided into a number of cells and each cell is served by an infrastructure element called the base station which works through a radio interface. The frequency license fees, real estate, distribution network and maintenance are the issues which dominates the cost for deploying a cellular network. Management of radio related resources is a critical design component in cellular communications. In RRM, we control parameters like Radio Frequency (RF) planning, link budgeting, modulation schemes, channel access schemes etc. RF planning includes cell planning, coverage of the network and capacity of the network. Our main focus in this thesis will be on cell planning and link budgeting and we will discuss them in context of a WiMAX network.

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Acknowledgements

All praises to ALLAH, the cherisher and the sustainer of the universe, the most gracious and the most merciful, who bestowed us with health and abilities to complete this project successfully.

This project means to us far more than a Mater degree requirement as our knowledge was significantly enhanced during the course of its research and implementation. We are especially thankful to the Faculty and Staff of School of Engineering Blekinge Institute of Technology (BTH) Karlskrona, Sweden, who have always been a source of motivation for us and supported us tremendously during this research.

We are extremely grateful to our project supervisor Professor Hans-Jürgen Zepernick who guided us in the best possible way in our project. He is always a source of inspiration for us. His encouragement and support never faltered.

We are also very thankful to all those course mates who have helped us morally as well as academically. In every hour of need, their full cooperation was there for us.

We special gratitude and acknowledgments are there for our parents for their everlasting moral support and encouragements.

Noman Shabbir & Hasnain Kashif Karlskrona, March 2009.

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LIST OF FIGURES

1.1 IEEE and ETSI standards for wireless networks ... 2

2.1 Examples of various MAC PDU frames ... 10

2.2 Over the air latency in a WiMAX network ... 12

2.3 WiMAX architecture ... 16

3.1 Different regions in the world ... 18

3.2 General architecture of cellular System ... 23

3.3 Hexagonal cells ... 24

3.4 Frequency Reuse of 1/4 ... 25

3.5 Cell site ... 25

3.6 Umbrella cell ... 26

3.7 Link budget ... 28

3.9 Comparison of FDMA, TDMA and CDMA ... 32

4.1 Frequency bands Used for WiMAX ... 34

4.2 Adaptive modulation scheme in a cell ... 41

4.3 BPSK constellation diagram ... 42

4.4 QPSK constellation diagram ... 42

5.1 Toolbox process flow chart ... 43

5.2 MatLab toolbox for link budget analysis in communication networks... 44

5.3 MatLab Toolbox for link budget analysis with some option ON ... 47

5.4 MatLab Toolbox for link budget analysis with all option ON ... 48

5.5 Path loss given by SUI propagation model for 2500 MHz ... 51

5.6 Path loss given by SUI propagation model for 3500 MHz ... 52

5.7 Path loss given by Ericsson propagation model for 2500 MHz ... 53

5.8 Path loss given by Ericsson propagation model for 3500 MHz ... 54

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5.9 Median attenuation factor and area gain ... 54

5.10 Path loss given by Okumura model ... 56

5.11 Geographical map of Lahore (Pakistan) ... 57

5.12 Nominal Cell Planning of Lahore City ... 57

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LIST OF TABLES

2.1 Different types of services provided by fixed and mobile WiMAX ... 5

2.2 Frequency bands and duplexing techniques for fixed WiMAX ... 6

2.3 Different parameters in WiMAX ... 8

2.4 Service class and applications ... 13

3.1 UMTS – TDD... 19

4.1 Frequency bands for WiMAX ... 35

4.2 System parameters for mobile WiMAX ... 37

4.3 OFDMA parameters for mobile WiMAX ... 38

4.4 Propagation model for mobile WiMAX ... 39

4.5 Link budget for mobile WiMAX ... 39

5.1 Different enviorments and path loss exponent ... 46

5.2 Different terrains and parameters ... 49

5.3 Propagation models and their path loss... 58

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C ^HAPTER 1

Introduction

ireless communication has brought a revolution in the world. There has been tremendous technological advancement in wireless networks in the past two or three decades. In the beginning of this new millennium we are having a new world which is the world of telecommunication. Telecommunication includes all the communication of computer networks, public telephone networks, radio networks, television networks

and internet.

Most of the communication networks used today are wireless in nature. Wireless means transferring the signals without wires using radio waves, infra red etc. In wireless networks there is unlimited mobility; we can access the network services from almost anywhere. In wired networks we have the restriction of using the services in fixed area.

The demand of wireless is increasing very fast as everybody wants to use the broadband services anywhere and anytime. The standardization of these wireless networks is also very important. The IEEE has developed many standards for different wireless networks.

Most common IEEE standards are IEEE 802.11 for WLAN, 802.15 for Bluetooth etc as shown in Figure 1.1 [1].

One of the most popular types of wireless networks is cellular networks. In cellular networks we divide the whole network into smaller cells and by this we can have more users in the network with better mobility and Quality of Service (QoS). Cellar networks are divided into different generations.

First generation (1G) cellular networks were introduced around 1980s. The first system was introduced in Japan. The first cellular network in Europe was built in Scandinavia in 1981 and it was known as Nordic Mobile Telephone (NMT). It uses the 450 MHz frequency band. This NMT system was also used in other parts of Europe as well. In America, Advance Mobile Phone System (AMPS) was used. The similarity

W

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WAN

MAN

LAN

PAN

3GPP, EDGE (GSM)

ETSI HiperLAN

ETSI HiperMAN

ETSI HiperPAN IEEE 802.15*

Bluetooth IEEE 802.11*

IEEE 802.16*

IEEE 802.20*

around 2.5 billion users. The purpose of this system was to have same system all over the world. GSM uses a frequency band of 900 MHz and 1800 MHz. GSM has many services like Short Messaging Services (SMS), Caller Identification, roaming etc. Enchantments were made in GSM when General Packet radio Service (GPRS) and Enhanced Data rates for GSM Evolution (EDGE) were introduced. These two systems increased the data rate in GSM. In America, CDMAone was used as a 2G technology.

Figure 1.1: IEEE and ETSI standards for wireless networks [1].

In the third generation (3G) cellular networks, Universal Mobile Telecommunication System (UMTS) was introduced. UMTS has higher data rates as compared to GSM while it enables more services like video conferencing, wireless television and wireless broadband as well. It has been designed to achieve the goal of global coverage. In America, CDMA 2000 was used as a 3G cellular technology.

The fourth generation (4G) of cellular networks is currently under development and hopefully it will be available in 2012. It will be having a data rate of at least 10 Mbps. It will be having more efficient spectral resources and other services as well.

WiMAX or Wireless Man is a 4G technology but some organizations refer it as a 3G technology. WiMAX is in implementation phase and it is the hottest wireless technology nowadays. WiMAX will be providing wireless internet anywhere and anytime.

1.1 Outline of Thesis and Contribution

Chapter 2

In this chapter, we discuss WiMAX, technical information and its architecture. WiMAX

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with 802.16d and mobile WiMAX having 802.16e standard. WiMAX is based on flexible and robust air interface. WiMAX physical layer is based on Orthogonal Frequency Division Multiplexing (OFDM) which is of the most effective technique. WiMAX Media Access Control (MAC) layer is also very flexible which can accommodate different traffic types such as voice, video and multimedia. WiMAX defines the network architecture which is based on Internet Protocol (IP). WiMAX also discuss QoS and mobility management issues.

Chapter 3

In this chapter, we discuss Radio Resource Management (RRM) which is very important in cellular networks. In cellular networks the available resources are very limited and we have to utilize them in the best possible manner. There should be an optimized solution to utilize the spectral efficiency. This will increase the overall coverage, capacity and QoS of the network. The main objective of radio resource management is to maximize the number of users in the network and minimize the cost of the network while the QoS should be there as well. In RRM, we control parameters like radio frequency planning, cell planning, link budgeting, modulation schemes, channel access schemes etc.

Chapter 4

In this chapter, we discuss RRM in context of WiMAX network. WiMAX is the technology of the future so RRM in WiMAX will be of great important. WiMAX will be using frequency bands of 2.5 GHz and 3.5 GHz in licensed and 5.8 GHz band in unlicensed regions. Cell planning in WiMAX is very important because coverage and capacity planning depends on it. Link budgeting is also very important because the transmitter power should be on a certain value so that the desired area should be covered and there must be enough power on the receiver side for effective communication. In mobile WiMAX, we require handovers and it will be using hard handovers. WiMAX will be using OFDM as a transmission scheme in which different modulation techniques are used.

Chapter 5

In this chapter, we discuss our simulation results. Our simulation is divided into two parts, link budget and cell planning. In the link budget part, we will focus on link budget in WiMAX. In the cell planning part, we will focus on cell coverage, nominal cell

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C ^HAPTER 2

WiMAX

he vision of broadband Internet access anywhere and anytime has appeared a distant dream, far from reality for the enormous majority of Personal Computers (PC), laptops and handheld users. WiMAX will be the most up to date wireless technology.

WiMAX systems are likely to convey wireless broadband access to residential areas and customers in economical way.

2.1 WiMAX Standards

WiMAX is having two different standards which are described below:

IEEE 802.16-2004

IEEE 802.16-2004 is often called IEEE 802.16d or fixed WiMAX. It uses OFDM and supports fixed and nomadic access in Non-Line of Sight (NLOS) and Line of Sight (LOS) environments. The WiMAX forum uses 3.5 GHz and 5.8 GHz frequency bands for it.

IEEE 802.16e

It is an amendment to 802.16-2004 and it is often called as 802.16e-2005 or mobile WiMAX. This version of WiMAX is designed to support mobility and dynamic mobile radio channels. On the bases of these modifications, it provides mobility and when we have mobility in a network then we need handovers and roaming. WiMAX IEEE 802.16e uses Scalable Orthogonal Frequency Division Multiple Access (SOFDMA) which is a multi-carrier modulation technique and it uses sub channelization. WiMAX forum has not announced the frequency bands. However, the 2.3 GHz and 2.5 GHz bands are supposed to be used for it [2].

WiMAX is a wireless technology which is optimized for the delivery of IP centric services over a large area. Its certification indicates interoperability of the equipment built to the IEEE 802.16 or compatible standard. WiMAX is a scalable wireless platform

T

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2.2 Introduction

WiMAX is a broadband wireless technology which supports different types of services like nomadic access, portable access, fixed and mobility. In the first type, IEEE 802.16- 2004 is optimized for fixed and nomadic access. These accesses are Point to Point (PTP) and Point to Multi-Point (PMP) applications. The second type is designed to support portability and mobility. Different types of services provided by WiMAX are described in Table 2.1.

Table 2.1: Different services provided by fixed and mobile WiMAX [2]

Definition Devices Locations, Speed

Handoff or

Handover 802.16-2004 802.16e

Fixed Access

Outdoor and indoor CPEs

Single,

Stationary No Yes Yes

Nomadic Access

Indoor CPEs, PCMCIA

cards

Multiple,

Stationary No Yes Yes

Probability

Laptops PCMCIA or

mini Cards

Multiple,

Walking Hard No Yes

Simple Mobility

Laptops, smart phones

Multiple, Low vehicular

Hard No Yes

Full Laptops, Multiple,

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The WiMAX forum has defined IEEE standard, spectrum bands, duplexing techniques and channel bandwidth for WiMAX. In IEEE 802.16-2004 standard, we use OFDM with 256 carriers while in IEEE 802.16e, we use SOFDMA. In many countries, the licensed spectrum band used for WiMAX is 3.5 GHz while 5.8 GHz is used as an unlicensed band. Time Division Duplex (TDD) and Frequency Division Duplex (FDD) are used as duplexing techniques in WiMAX. In Table 2.2, different frequency bands and duplexing techniques used for fixed WiMAX are described.

Table 2.2: Frequency bands and duplexing techniques for fixed WiMAX [2]

Frequency (GHz) Duplexing Channels Bandwidth (MHz)

IEEE standard

3.4-3.6 TDD 3.5 802.16-2004

3.4-3.6 FDD 3.5 802.16-2004

3.4-3.6 TDD 7 802.16-2004

3.4-3.6 FDD 7 802.16-2004

5.725-5.85 TDD 10 802.16-2004

The channel bandwidth is highly dependent on the frequency band allocated by regulators. The channel bandwidth is limited to 3.5 MHz and 7 MHz in the licensed band [2].

2.3 Overview of Technical Information

WiMAX is the name of a wireless broadband technology which describes interoperable implementations and different standards of IEEE 802.16. Here, we will discuss some technical information of WiMAX.

2.3.1 WiMAX Physical Layer

WiMAX physical layer uses OFDM as a transmission technique. By using OFDM as a transmission scheme, we have higher data rates as compared to GSM and UMTS.

2.3.2 OFDM Basics

The OFDM technology belongs to the family of multicarrier modulation transmission schemes. The idea is that, we divide the given data stream which has higher bit rates into parallel lower bit rate data streams. After that different subcarriers are used to modulate each individual data stream. These carriers are called subcarriers. By using this technique, we minimize the Inter-Symbol Interference (ISI). The symbol time can be increased if delays are introduced in the channel. With the utilization of this method,

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delays are introduced in all wireless channels. These channels are only around ten percent of symbol duration time. Consequently, in higher data rate systems, the symbol duration is small. The symbol duration of every stream is increased by dividing the data stream into many parallel streams. Hence, the delay spread is very small when compared to the symbol duration time.

This modulation scheme is a proficient description of multicarrier modulations in which the selected subcarriers lie orthogonal to each other over the duration time of symbols. In result, there will be minimum inter-carrier interference which is caused by the overlapping of subcarriers. There is proper interspacing between these subcarriers as well. Subcarrier bandwidth is

BSC = B / L (2.1) where B is the nominal bandwidth and it is same as the data rate. While L denotes number of subcarriers and it makes sure that all carriers lie orthogonal to each other over the duration of symbol time.

2.3.3 WiMAX OFDM Parameters

Fixed WiMAX and Mobile WiMAX have very little differences in implementing the OFDM physical layer. Fixed WiMAX or IEEE 802.16-2004 uses OFDM on the physical layer which has a 256 Fast Fourier Transform (FFT) size while Mobile WiMAX or IEEE 802.16e standard uses SOFDMA based physical layer. In Mobile WiMAX, the FFT size is variable it can vary from 128 to 2048 bits. In fixed and mobile WiMAX, different parameters of OFDM and SOFDMA are used on the physical layer. These different parameters are described in Table 2.3 [3].

In IEEE 802.16 standard, it was stated that the physical layer of WiMAX will be operating in the range of 10 to 66 GHz frequency bands. Firstly, IEEE 802.16a was updated in 2004 as 802.16-2004 and it was decided that it will be working on 2 to 11 GHz frequency bands. Then it was again updated as 802.16e-2005 in 2005 and it was also decided that WiMAX 802.16e will be using SOFDMA. In the advanced version 802.16e, we use Multiple Input Multiple Output (MIMO) system having multiple antennas on both the transmitter and receiver sides. The potential benefits of this advanced version are better coverage, self installation, less power consumption, frequency reuse and spectral efficiency.

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Table 2.3: Different parameters in WiMAX [3]

Parameter Fixed WiMAX OFDM Mobile WiMAX

Scalable OFDMA

FFT size 256 128 512 1024 2048

Number of used data

subcarriers 192 72 360 720 1440

Number of pilot subcarriers 8 12 60 120 240

Number of null/guard band

subcarriers 56 44 92 184 368

Cyclic prefix or guard time

(Tg/Tb) 1/32 1/16 1/8 ¼

Oversampling rate (Fs/BW)

Depends on bandwidth: 7/6 for 256 OFDM, 8/7 for multiples of 1.75MHz, and 28/25 for multiples of

1.25MHz, 1.5MHz, 2MHz, or 2.75MHz.

Channel bandwidth (MHz) 3.5 1.25 5 10 20 Subcarrier frequency

spacing (kHz) 15.625 10.94 Useful symbol time (ms) 64 91.4 Guard time assuming 12.5%

(ms) 8 11.4

OFDM symbol duration

(ms) 72 102.9

Number of OFDM symbols

in 5 ms frame 69 48.0

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2.3.4 WiMAX MAC Layer

WiMAX MAC layer is working as an interface between the WiMAX physical layer and upper transport layers. The MAC Service Data Units (MSDUs) are the packets which MAC layer takes from the upper layers. Then for transmission these packets are converted into MAC Protocol Data Units (MPDUs). In MAC layer design a convergence sublayer is included which can be used as an interface to higher layer protocols like Asynchronous Transfer Mode (ATM), Time Division Multiplexing (TDM), Voice, Ethernet and IP. At this time, WiMAX only supports IP and Ethernet but it provides mapping information to the higher layers. The convergence sublayer of WiMAX physical layer supports the MSDU header control to decrease the overhead load on each packet which is caused by the upper layers [3].

WiMAX MAC layer uses a variable length MPDU and it also provides flexibility for their efficient transmission. For instance, if we want to save physical layer overhead, we can add multiple MPDUs of same or different lengths to a single burst. Similarly, we can save MAC header overhead by adding multiple MSDUs which belong to the same higher layer protocol into a single MPDU. On the other hand, large MSDUs can be fragmented into smaller MPDUs and after that it can be sent across multiple frames.

In Figure 2.1, different examples of MAC Packet Data Unit (PDU) frames are shown. Every MAC frame contains a Connection Identifier (CID) and every MAC frame starts with a Generic MAC Header (GMH). The length of frame depends on Cyclic Redundancy Check (CRC), sub-headers and payload. This payload can be a transport message or a management message. This transport payload may include bandwidth requests or retransmission requests. The type of transport payload is recognized by the sub-header that immediately follows it. MAC layer also supports Automatic Repeat Request (ARQ). By using ARQ, unfragmented and fragmented MSDUs can be retransmitted in case of loss. The maximum length of the frame is 2,047 bytes [3].

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GMH Other SH

Packed Fixed size

MSDU

Packed Fixed size

MSDU

…

Packed Fixed size

MSDU

CRC

(a) MAC PDU frame carrying several fixed length MSDUs packed together

GMH Other SH FSH MSDU Fragment CRC

(b) MAC PDU frame carrying a single fragmented MSDU

GMH Other SH PSH

Variable size MSDU or Fragment

PSH

CRC

(c) MAC PDU frame carrying several variable length MSDUs packed together

GMH Other SH ARQ Feedback CRC

(d) MAC PDU frame carrying ARQ payload

GMH Other SH PSH ARQ

Feedback PSH

CRC

(e) MAC PDU frame carrying ARQ and MSDU payload

GMH ARQ Feedback CRC

(f) MAC management frame

Figure 2.1: Examples of various MAC PDU frames [3].

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2.3.5 Channel Access Mechanisms

The WiMAX MAC layer is completely responsible for assigning bandwidth for all uplink and downlink users at the Base Station (BS). This is the only time when the BS has all the control over bandwidth allocations. During this period, multiple users are trying to connect or already connected with the BS. In this case, BS assigns bandwidth to all the users. Then it is distributed on different connections. The BS is also responsible for controlling all the downlinks and uplinks by the users. The BS may allocate bandwidth to each Mobile Station (MS) for downlinks. It is based on the requirement of the incoming traffic. The MS can also request bandwidth allocations for uplinks.

The WiMAX standard supports several mechanisms by which an MS may request and get uplink bandwidth, depending on specific QoS and traffic parameters related to the required service. BS assigns shared resources from time to time to each MS, which can be used for different services. This process is called as polling. Polling may be done either one by one which is called unicast or in groups which is called multicast. When we have insufficient bandwidth to poll every MS one by one then multicast polling is used.

In multicast polling, every polled MS tries to use a dedicated or shared slot. WiMAX gives a resolution mechanism for the scenario where more than one MS are trying to utilize the dedicated or shared slots. The MS can request for more bandwidth in many ways. For instance, by sending MPDU bandwidth request alone, the ranging channel can be used as well for sending a bandwidth request and piggybacking the bandwidth request on generic MAC packets [3].

2.3.6 QoS in WiMAX

The QoS is determining the fact that how successfully a wireless technology can transfer high value services like voice and video. For good QoS, the latency, jitter and packet loss should be minimized. After solving these issues, a good QoS is guaranteed. WiMAX has a very low latency across the wireless period. Most vendors have products in which we have latency less than 10 ms from the transmitter to receiver. The latency is measured from end to end. For instance, Voice over IP (VoIP) is highly vulnerable to latency. If the latency increases to 150 ms, the quality of the call begins to decrease. If the latency is equal to or above 200 ms, a call or conversation would be having very low quality [4].

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Figure 2.2: Latency in the air of a WiMAX network [4].

In WiMAX, the large bulk of latency does not occur on the air link between subscribers and base station. This latency can be introduced on the wired portion of the connection between the subscriber and whatever the "other end" could be, for instance, it can be a web server, IPTV server or VoIP calls. Figure 2.2 illustrates the latency on the wireless portion in a WiMAX network.

WiMAX also describes an idea of a service flow. The unidirectional flow of packets with a specific set of QoS parameters is called service flow and it is expressed by a service flow identifier (SFID).Different SFIDs and applications are shown in Table 2.4.

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Table 2.4: Service class and applications [4]

Service Class Applications QoS Specification

Unsolicited Grant Services

(UGS) VoIP

Maximum sustained rate, Maximum latency

tolerance, Jitter tolerance

Best-effort service (BE) Web browsing, data transfer

Maximum sustained rate, Traffic priority

Real-time Polling service

(rtPS) Streaming audio and video

Minimum reserved rate, Maximum

sustained rate, Maximum latency

tolerance, Traffic priority

Non-real-time Polling service

(nrtPS)

FTP

Minimum reserved rate, Maximum sustained rate, Traffic

priority

Extended real-time Polling service

(ErtPS)

VoIP (voice with activity detection)

Minimum reserved rate, Maximum

sustained rate, Maximum latency

tolerance, Jitter Tolerance, Traffic

priority

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2.3.7 WiMAX Mobility Support

In fixed broadband access, WiMAX distinguishes four mobility related usage circumstances:

Nomadic: In this scenario, a fixed location is allocated to the user to utilize the services. In order to connect from a different location, a break or disconnection is supposed to be faced by the user in this case.

Portable: Employing nomadic access with a portable device, the portability is ensured for the user at the cost of handovers.

Simple mobility: Almost uninterrupted handovers are achieved besides attaining a speed of 60 km/h by the user with very short time intervals (ranging up to few milliseconds).

Full mobility: The user can move up to 120 km/h speed. Mobility and seamless handovers with less than 50 ms latency and < 1 % packet loss is supported.

WiMAX network supports both fixed and nomadic applications. However, an extension in support for portability and full mobility can be achieved in addition, thereafter. IEEE 802.16e-2005 standard describes a structure for supporting mobility management. The Standard describes signaling mechanisms for tracking subscriber stations which are moving from one BS coverage range to another in active mode. The IEEE standard has protocols to allow a seamless handover of enduring connections from one BS to another. The WiMAX Forum has used the structure defined in IEEE 802.16e- 2005 to additionally make network architecture structure such that it supports mobility management. This architecture facilitates the IP layer mobility using mobile IP.

IEEE 802.16e-2005 describes three types of handovers. First is mandatory called Hard Handover (HHO). In HHO, first the connection from the source cell is terminated and then it is connected to the target cell. During the handover, the data is stored in a buffer for very short time. The handover decisions are made by the BS normally on the bases of relative signal strength. The second handovers method is called Fast Base Station Switching (FBSS). In FBSS, MS maintains a valid connection at the same time with more than one BS. The last type of handovers is called Macro Diversity Handover (MDHO) with multiple connections to different BS.

2.3.8 Multicast and Broadcast Services

This MAC layer handles the Multicast and Broadcast Services (MBS). MBS have function and features in the standard which are described as

• The MS has a signaling system that requests and after confirmation launch the multicast and broadcast services.

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• The subscriber station access the multicast and broadcast services over a single/multiple BS depending upon its capability and desire.

• Multicast and broadcast services are associated with QoS and encrypted by a globally describe traffic encryption key.

• There are different methods to deliver multicast and broadcast services traffic to idle mode subscriber stations.

• MAC layer supports the macro diversity to enhance the delivery performance of multicast and broadcast services traffic.

2.4 WiMAX Features for Performance Enhancement

WiMAX describes the advanced features of improving the performance. These advanced features which can be supported are Advanced Antenna Systems (AAS), hybrid ARQ and frequency reuse. However, the former two will be elaborated in detail under this section and the later (frequency reuse) will be discussed in the next chapter.

2.4.1 Advanced Antenna Systems

IEEE 802.16 standard provides the support for implementing advanced multi-antenna solutions which improve the system performance. We are deploying AAS to get gain in the capacity of the system and more bandwidth efficiency. The varieties of multiple antennas mentioned in AAS are as follows:

Transmit diversity: For transmit diversity, there may be two or more antennas on the transmitter and receiver sides. A numbers of space time block coding schemes can be employed to achieve transmit diversity for the downlink.

Beamforming: Beamforming ensures better capacity, more coverage and greater reliability in the network.

Spatial Multiplexing: Different streams are transmitted on multiple antennas in spatial multiplexing. As multiple antennas are provided at the receiving end, the space time processing separates the stream. Hence, improved data rate and capacity of the system can be attained applying multiple antennas [3].

2.4.2 Hybrid ARQ

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the received block, the retransmission is requested. The received block is combined with the last detected code and serves as the input to the FEC decoder, thereafter. There are two types of Hybrid ARQ, the Type I is called Chase combining and Type II is called Incremental redundancy [3].

2.5 WiMAX Network Architecture

The information about the air interface is specified in the mobile WiMAX standard but it seems lying unspecified about the end-to-end WiMAX network requirements. A network reference model has been designed by WiMAX Network Working Group (NWG). The deployment of WiMAX can be accomplished by employing this architecture. With the help of this framework, we can ensure interoperability among different WiMAX components and operators. A logical division for a WiMAX network into three different parts can be interpreted as Mobile Stations (MS), Access Service Network (ASN) and Connectivity Service Network (CSN). Figure 2.3 shows important functional entities of a WiMAX network [3].

IP Network Access

Network BS

BS

ASN- GW

Access Service Network (ASN) MS

MS

AAA

MIP-HA

OSS/BSS

Gateway

ASP

Connectivity Service Network (CSN)

Internet

IP Network

PSTN

3GPP/

3GPP2 AAA: Authentication, Authorization, Accounting

ASN-GW: Access Service Network Gateway MIP-HA: Mobile IP Home Agent OSS: Operational Support System

MS: Mobile Station BS: Base Station

Figure 2.3: WiMAX network architecture [3].

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C ^HAPTER 3

Radio Resource Management

adio Resource Management (RRM) is a control mechanism for the overall system which is used to manage radio resources in the air interface within a cellular network. The main objective of our study of RRM is to analyze and determine a way to utilize the available spectral resources as efficiently as possible. The frequency license fees, real estate cost, distribution network and maintenance are the issues which dominates the cost of deploying a cellular network. Hence, the limitation of spectral recourses is being faced due to the above mentioned factors regarding their cost effectiveness. The core objective of this research is to serve the purpose of maximization of the performance along with spectral efficiency enabling maximum number of users in our network and to bring QoS up to the mark.

RRM is of vital importance in cellular systems due to the bandwidth limitations.

In RRM, point to point capacity issues are not considerable whereas it suitably tackles multi-cell traffic problems. For cellular systems, the whole network is geographically divided into smaller geometric regions called cells. In each cell, there is a network infrastructure element called BS. These BS are connected to each other and to the core network via radio interface. The overall number of users (traffic) and performance of the whole network depends upon the individual cells performance. The optimum utilization of this radio interface is of key important. An efficient RRM algorithm is a basic requirement for it, as efficient RRM techniques enhance the overall capacity of the system and decrease the cost effectively [5]. RRM involves techniques and algorithms for controlling parameters which are as follows:

• Frequency Band Allocation

• Cell Planning

R

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• Multiple Access Schemes

3.1 Frequency Band Allocation

Frequency band allocation is one of the most important parameters of RRM. The world is lacking global standards for cellular communications. In different regions of the world even a single frequency band is used for different cellular systems. Almost every country has its own regulatory authority. For instance, Federal Communications Commission (FCC) is the authority which is responsible for the allocation of frequency bands in the US. International Telecommunication Union (ITU) is a United Nation based organization which is working to standardize the frequency bands all over the world [7]. It is very important that we have the same frequency band all over the world for one technology as it would be easier and convenient for a user to use the same equipment and technology anywhere.

Due to historical and political reasons, there are different frequency bands allocated for cellular communication systems in US, Europe and Asia. The world is divided into three regions and all the three regions have different frequency bands which are illustrated in Figure 3.1 below [6].

Region 1 consists of Europe, Africa, Russia and some parts of the Middle East.

Regions 2 covers USA, Canada and all the Latin America while in Region 3 China,

Figure 3.1: Different regions of frequency allocation in the world [6].

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Almost every cellular technology has different frequency bands in these three regions.

But even in one region there are different frequency bands for same technologies. In Table 3.1 different frequency bands in different regions for UMTS are shown [8].

Table 3.1: UMTS - FDD [8]

Operating Band

Frequency Band

Up Link Frequencies

UE Transmit

(MHz)

Down Link Frequencies

UE Receive (MHz)

Channel Number (UARFCN)

UL

DL

I 2100 1920 –

1980

2110 – 2170

9612 – 9888

10562 – 10838

II 1900 1850 –

1910

1930 – 1990

9262 - 9538 additional

12, 37, 62,87, 112,

137, 162, 187, 212, 237, 262,

287

9662 - 9938 additional 412, 437, 462,487, 512, 537, 562, 587, 612, 637, 662, 687

III 1800 1710 –

1785

1805 –

1880 937 – 1288 1162 – 1513

IV 1700 AWS 1710 – 1755

2110 – 2155

1312 - 1513 additional 1662, 1687, 1712, 1737,

1537 - 1738 additional 1887, 1912, 1937, 1962,

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Table 3.2: UMTS - FDD Continued [8]

UE Transmit

(MHz)

UL

DL

V 850 824 – 849 869 – 894

782, 787, 807,812, 837, 862

4357 - 4458 additional 1007, 1012,

1032,1037, 1062, 1087

VI 800 830 – 840 875 – 885

812, 837

4387 - 4413 additional 1037, 1062

VII 2600 2500 –

2570

2620 – 2690

2012 - 2338 additional 2362, 2387, 2412, 2437, 2462, 2487, 2512, 2537, 2562, 2587, 2612, 2637, 2662, 2687

2237 - 2563 additional 2587, 2612, 2637, 2662, 2687, 2712, 2737, 2762, 2787, 2812, 2837, 2862, 2887, 2912

VIII 900 880 – 915 925 – 960 2712 – 2863

2937 – 3088

IX 1800 1749.9 -

1784.9

1844.9 – 1879.9

8762 – 8912

9237 – 9387

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Table 3.3: UMTS - FDD Continued [8]

UE Transmit

(MHz)

UL

DL

X 1700 1710 –

1770

2110 – 2170

2887 - 3163 additional 3187, 3212, 3237, 3262, 3287, 3312, 3337, 3362, 3387, 3412, 3437, 3462

3112 - 3388 additional 3412, 3437, 3462, 3487, 3512, 3537, 3562, 3587, 3612, 3637, 3662, 3687

XI 1500 1427.9 -

1452.9

1475.9 – 1500.9

3487 – 3587

3712 – 3812

XII 700 698 – 716 728 – 746

3612–3678 additional 3702, 3707, 3732, 3737, 3762, 3767

3837–3903 additional 3927, 3932, 3957, 3962, 3987, 3992

XIII 700 777 – 787 746 – 756

3792–3818 additional 3842, 3867

4017–4043 additional 4067, 4092

3892–3918 4117–4143

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UMTS/HSDPA/HSUPA frequency bands deployment

• “Band I (W-CDMA 2100) is used in Europe, Africa (ITU Region 1), Asia, Oceania and Brazil (part of ITU Region 2).

• Band II (W-CDMA 1900) is used in North America and South America (ITU Region 2).

• Band IV (W-CDMA 1700 or Advanced Wireless Services) is used in the United States by T-Mobile.

• Band VIII (W-CDMA 900) is used in Europe, Asia and Oceania (ITU Region 1 and ITU Region 3).

• Band V (W-CDMA 850) in used in Australia, Brazil, Canada, USA ,other parts of South America and some parts of Asia (ITU Region 2 and ITU Region 3).

• Band XII - XIV were added for future use in the 700MHz spectrum auctioned in the United States and Canada in early 2008.” [8]

Acquisition of a frequency license is an issue to be addressed at the forth most priority while deploying a cellular network. Higher costs have to be paid by the companies to the local regulatory authorities for this purpose. The overall bandwidth of the system depends on that frequency band. The cellular companies do not have a choice for the acquisition of frequency band as the authorities have predefined frequency bands for different cellular technologies. Hence, to make an efficient use of the provided band of frequency, the role of employing RRM emerges as of key importance.

3.2 Cell Planning

The development of cellular networks has brought a revolution. In this modern era, mobile telephones, personal communications, wireless broadband internet and other applications widely use cellular networks. In cellular networks, the whole geographical area is divided into smaller geometric parts which are termed as cells. A Base Transceiver Station (BTS) or BS is installed in each cell of a cellular network with a low power transmitter. Each cell has its own transmitter, receiver and control unit. These base stations are connected to each other and to the Mobile Switching Centre (MSC) via radio interface. The MSC is connected to the Public Switched Telecommunication Networks (PSTN). The basic overview of cellular system is shown in Figure 3.2 [9].

The whole procedure of dividing the network into cells and placement of base stations in these cells is called cell planning. Cell Planning is usually applied before deploying the network, increase in the capacity of the system can also be achieved

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way to achieve an efficient and optimum condition as the base stations are also to be installed accordingly. Cell planning also includes call traffic capacity. As the number of users in a cell increases, smaller cells can be created and there is an increase in the overall capacity of the network as a result. The number of cells is required to be optimum and QoS up to the standard. The installation cost for a BS is very high as it also includes the cost or rent of the real estate.

BTS

BTS Mobile

Telecomm- unication Switching

Center (MSC) PSTN

Figure 3.2: General architecture of cellular system [9].

The cells used in the planning of cellular communication are hexagonal in shape.

The hexagons are equidistant from the centers, making them preferable to be adopted in cellular networks. The hexagons are shown in Figure 3.3 [10]. In practice an ideal hexagonal does not exist. The distance between a cell and adjacent cell is

d = 3 R (3.1) where R is the radius of cell.

Area of one cell = A = 1.5 3 R. (3.2) Total Area of network = Number of cells x Area of one Cell. [9] (3.3)

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3.2.1 Cell Site

Cell site is a very common term which describes the location or site where the base station is deployed. Cell site or BS is consisted of a tower, mounted transmitter and receiver antennas, power supply, the backup power supply and controlling element which is commonly termed as cell Base Station Controller (BSC). The model of a cell site is shown in Figure 3.4 [12].

A cell site is commonly known as BS or BTS. These BTS are connected to each other via optical fiber or microwave links. BTS consists of two parts, BSC and Radio Network Controller (RNC). These BSCs are connected to Mobile Telephone Switching Centre (MTSC) or MSC.

Figure 3.3: Hexagonal cells [10].

The most important parameter which is concerned with the cell site is the range of cell site. The range of the cell site depends upon the transmitter power, size of antenna, antenna height, the direction of antenna and geographical position of the cell site.

3.2.2 Frequency Reuse

Frequency reuse means that same frequencies can be used in a different area for different communications. The capacity of the system can be increased by using frequency reuse.

A problematic situation in this regard is to determine the frequency reuse interval that is to specify the number of cells after which we can reutilize the same frequency again as the same frequency reuse introduces interference. Every cell is being allocated a number of fixed frequency channels. If we have a total number of cells N and number of available frequency channels assigned to a cell is K then the frequency reuse factor is K / N.

Frequency reuse procedure is shown in Figure 3.5 [13].

d

d d

d

d R

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Figure 3.4: Cell site [12]. Figure 3.5: Frequency reuse of 1/4 [13].

3.2.3 Types of Cells

As the number of users increases in a single cell, the capacity of the system is overloaded and the QoS is affected. This problem is solved by dividing the cell into smaller cells.

Below are some types of cells regarding the size of cell, power and height of antenna.

• Macro Cells

• Micro Cells

• Nano Cells

• Pico Cells

• Umbrella Cells

• Sectorized or Selective Cells [14]

1) Macro Cells

A common type of cells which is implemented in cellular communication is macro cells.

These are the largest cells and cover an area of more than ten kilometers. These cells are usually used in rural and mountainous regions as we do not have a lot of traffic in these regions. In this cell, the BS is usually placed on the top of a building or on the top of a mountain.

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Micro cells are commonly used in urban areas. The coverage area for micro cells is up to a few kilometers. In these cells, the BS is required to be placed at the roof of the building, about 10 to 20 meters above.

3) Nano Cells

In cities with densely populated areas, micro cells are replaced by smaller cells which are called nano cells. The geographical area covered by these cells is less than a kilometer nearly a few hundred meters. The BS in this case needs to be placed at the top of a roof but not very much higher.

4) Pico Cells

Pico cells are the smallest cells used in cellular networks. It has a coverage area of few meters, nearly 30 to 80 meters.

Figure 3.6: Umbrella cell [14].

5) Umbrella Cells

An umbrella cell consists of a macro cell and several micro cells as shown in Figure 3.6 [14]. Umbrella cells are used when a mobile station is moving at a fast speed. In case of micro cells, a lot of handovers will be required. To avoid this situation, this user is connected to a macro cell, covering the whole area and in this case, the mobile user remains in the same cell for a longer time period. By using this method, number of handovers can be limited. An umbrella cell is a very good practical example of RRM.

6) Sectorized or Selective Cells

Usually, a cell has coverage in all directions because we use three antennas on a single BS with 120 degree apart from each other. Thus, the coverage is almost 360 degree, means almost equal in all directions. In some cases, we don’t require coverage in all

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directions. For instance, in case of a tunnel, the coverage is only required in one direction. These types of cells are known as sectorized or selective cells.

3.3 Link Budget

Communication networks such as mobile telephony, satellite communications and Bluetooth connectivity is very important in our daily lives. Therefore, the estimation of the transmitted signal power in the radio channel and the achievement of appropriate signal coverage area with acceptable QoS have become significant for communication systems. Link budget is the estimation and determination of a transmitted signal upon reaching the receiving end. Link budget encapsulates a significant importance while doing research work or designing in the field of communication engineering.

Determining and adding all the gains and losses in a communication system is called the link budget. Link budget specifies the amount of transmitter power needed to transmit a signal with a certain Signal to Noise Ratio (SNR) and acceptable Bit Error Rate (BER). Path loss, noise, rain loss, connectors or cable losses, receiver sensitivity and antenna gains are the factors which are required to be taken into the account while estimating the link budget. Figure 3.8 illustrates the link budgeting procedure [15].

3.3.1 Calculation of Link Budget

Link budget is very important part of the network planning process. Link budget is a process for indoor and outdoor applications. Link budget gives the measurements which are required for the coverage, capacity and QoS requirement of the network. In cellular networks, there are two types of link budget calculations such as:

• Link from the BS to the Subscriber Station (SS)

• Link from the SS to the BS

The Link budget adds all the gains and losses to the transmitter power (in dB) to the received power. In the link budget we can calculate the following parameters:

• Path loss

• Antenna gain

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• Attenuation (rain or fog)

• SNR

Transmitter Receiver

Prx = Ptx + Gtx – Apl + Grx - Am

Prx = Received power (dBm) Ptx = Transmitter power (dBm) Gtx = Transmitter Antenna Gain

(dBi)

Grx = Receiver Antenna Gain (dBi)

Apl = Path loss (dB) Am = miscellaneous losses

Figure 3.7: Link budget [15].

3.4 Call Admission Control

An efficient RRM algorithm includes call admission control as well. Call admission control involves both new and handover calls. Handover or Handoff is a procedure in which an ongoing call or data session is transferred from one cell to another cell due to the movement of mobile user. But in this transfer the QoS, good voice quality and fast data transfer is required to be maintained.

In cellular communication, the network is divided into cells and each cell is assigned with some fixed frequency channels. Every time when a new call or data session is originated in a cell or a handover call is transferred to the same cell, a free channel is assigned to this call. As every cell has fixed number of channels so it limits the capacity of the cell. A call or data session is blocked or dropped in case of preoccupation of all the channels. Every customer or user wants to be connected at all the times to use different services. For these users call drop or blocking is unacceptable and it becomes an issue of QoS. Hence, an efficient utilization of these frequency channels is expected.

3.4.1 Handovers

Handover as discussed earlier is a criterion for retention of an ongoing call from one cell to another cell when the user is moving in the range of different cells. If the handover

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criterion is not specified, the probability for a call to be dropped increases extensively.

Some important algorithms used for handovers are discussed below:

Non Prioritized Handover Scheme (NPS)

This scheme assigns same priority to all the calls in a particular cell that is the equal priority for new calls and handovers. When a new call is initiated in a cell or a handover is transferred to that cell, a free channel is automatically assigned to it. In this scheme, all the channels are available for new or handover calls. A limitation with this scheme arises in a case when all the channels are occupied and no more new calls can be made in that cell. Hence, the user is required to move into another cell to make a call. Similarly, if all the channels are busy in a cell and a handover is transferred to that cell, the call will be dropped due to unavailability of free channels.

Reserved Channel Scheme (RCS)

In RCS, some frequency channels are particularly reserved for handover calls. If there are total number of channels available ‘N’ and some number of the ‘N’ channels ‘S’ are reserved for handovers. The reserved ‘S’ channels are only used for handover. In this scenario, when handover occurs, it is automatically assigned a reserved channel even all the other channels are occupied. These reserved channels are never assigned to new calls.

The number of reserved channels can vary in different cellular networks.

3.4.2 Types of Handover

There are two types of handovers which are currently being used in cellular networks which are as follows:

Hard Handovers

In hard handovers, the channel in the source cell (from where the call originates) is released first and after that the channel in the target cell (where the call is transferred) is engaged. Disconnection from the source cell takes place first and the target cell is connected thereafter. These types of handovers are also known as Break Before Make (BBM). In this type, the data is temporarily stored in a register during the handover. This data is once utilized after the handover and it is never used again. This type is advantageous in the sense that at anytime only one channel is used for one call. A

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Soft Handovers

In this case, the channel in the source cell is retained for a while along with the channel in the target cell. The connection to the target cell is first established before the termination of the connection from the source cell. In this way, the user is connected with two cells simultaneously. These types of handovers are also known as Make Before Break (MBB).

The significance of this scheme relies in the fact that the channel in the source cell is only released when a reliable connection in the target cell is established. Hence, the call drop rate decreases in this scenario. On the other hand, it is wastage of resources to use two channels simultaneously for one call. The hardware required for soft handovers is costly and complex as it must be compatible to engage both channels simultaneously.

Intra-cell handovers

In this type of handovers, source and the target cell are the same. The change of channel occurs only in case of interference in the channel. These types of handovers are called as intra-cell handovers.

3.5 Modulation Schemes

In wireless networks, different modulation schemes are used for different types of communications. In RRM perspective, the selection of an efficient modulation scheme is of significant importance. In modulation, a baseband signal (low frequency signal) is multiplied with a carrier signal or modulator signal (normally a high frequency signal).

The signal can be transmitted for longer ranges by using modulation. A carrier signal is normally a sinusoidal signal of high frequency. At the receiver end, the signal is then demodulated with the help of a device called demodulator. There are two types of modulations, analog modulations and digital modulations.

3.5.1 Analog Modulations

In analog modulations, the carrier signal is continuously multiplied with the source signal. The source signal is usually a continuous signal. This type of modulation is normally used to transfer analog signals. Following are some important types of analog modulations [16]:

• Amplitude Modulation (AM)

• Frequency Modulation (FM)

• Phase Modulation (PM)

• Quadrature Amplitude Modulation (QAM)

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3.5.2 Digital Modulations

In digital modulations, a digital signal is multiplied with a carrier signal. Some important types of digital modulation are as follows [16]:

• Frequency Shift Keying (FSK)

• Phase Shift Keying (PSK)

• Multi-frequency Shift keying (MFSK)

• Binary Phase Shift keying (BPSK)

• Quadrature Phase Shift keying (QPSK)

• Differential Phase Shift Keying (DPSK)

• Quadrature Differential Phase Shift Keying (QDPSK)

• Gaussian Minimum-shift keying (GMSK)

• Orthogonal Frequency Division Multiplexing (OFDM)

3.6 Multiple Access Techniques

In cellular communication, numbers of channels in a cell are limited. Hence, the capacity of the whole network is limited. Thus, multiple access techniques are of great importance.

Multiple access implies the utilization of service by several users at the same time. A frequency channel can be shared among many users at the same time and the capacity of the network can be increased. These multiple access techniques are used on the physical layer. Some important multiple access techniques are described below:

3.6.1 Frequency Division Multiple Access (FDMA)

In this channel access scheme, the whole frequency band is divided into smaller frequency bands and each user is assigned a separate frequency band. These frequency bands are allocated to the users by ensuring minimum interference between different frequency bands.

3.6.2 Time Division Multiple Access (TDMA)

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frequency band. TDMA is used in 2G cellular networks while a combination of TDMA and FDMA is used in GSM.

Figure 3.9: Comparison of FDMA, TDMA and CDMA [17].

3.6.3 Code Division Multiple Access (CDMA)

CDMA scheme adopts the method for transmission of data of multiple users simultaneously with a code. In FDMA, the bandwidth is divided among the users while in TDMA, the time is divided among the users whereas in CDMA, both time and frequency are divided. The difference between FDMA, TDMA and CDMA is illustrated in Figure 3.9 [17].

3.6.4 Orthogonal Frequency Division Multiple Access (OFDMA) OFDMA is a new scheme and it is based on OFDM. In this scheme, the carrier signal is divided into smaller subsets and a subset of carrier signals is used to transmit the data of a single user. OFDMA can be used alongside with OFDM to increase the QoS of the system.

User 1

User 2 User 3

User 1 User 2 User 3

TCDMA = KT4 T4 = LTCHIP

User 1

User 2 User 3

1 / T^CHIP

T⁴

1 / L

1 / Td

1 / Td

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C ^HAPTER 4

Radio Resource Management in WiMAX

iMAX is an extension of Wireless LAN 802.11 (Wi-Fi) and it will be providing wireless broadband services. WiMAX is a possible replacement of Digital Subscriber Line (DSL). WiMAX is divided into two standards, fixed WiMAX and mobile WiMAX. In fixed WiMAX, the user is having a fixed location while in mobile WiMAX, users are provided with mobility to move around. Fixed WiMAX is currently in deployment phase in many parts of the world while mobile WiMAX is in planning phase and it will be implemented in few years.

In the previous chapter, RRM was discussed in perspective of general cellular networks. Here, the focus will be on RRM in WiMAX networks. The architecture of WiMAX network is similar to the general cellular networks architecture. Hence, RRM is of great importance in WiMAX as well. The main function of RRM is to ensure the QoS.

The optimal solution in designing and planning a network is critical due to the limitation of resources. Different parameters of RRM in WiMAX will be discussed here which are as follows:

• Frequency Band Allocation of WiMAX

• Cell Planning in WiMAX

• Link Budget of WiMAX

• Call Admission Control in WiMAX

RADIO RESOURCE MANAGEMENT IN WiMAX

RADIO RESOURCE

MANAGEMENT IN WiMAX

Noman Shabbir Hasnain Kashif

ABSTRACT

Acknowledgements

TABLE OF CONTENTS

LIST OF FIGURES

LIST OF TABLES

C HAPTER 1

Introduction

W

1.1 Outline of Thesis and Contribution

C HAPTER 2

WiMAX

2.1 WiMAX Standards

T

2.2 Introduction

2.3 Overview of Technical Information

2.4 WiMAX Features for Performance Enhancement

2.5 WiMAX Network Architecture

C HAPTER 3

Radio Resource Management

R

3.1 Frequency Band Allocation

3.2 Cell Planning

3.3 Link Budget

3.4 Call Admission Control

3.5 Modulation Schemes

3.6 Multiple Access Techniques

C HAPTER 4

Radio Resource Management in WiMAX

W

C ^HAPTER 1

C ^HAPTER 2

C ^HAPTER 3

C ^HAPTER 4